The present invention relates generally to actuator control techniques, and more particularly to the control of force sensations output by actuators in a force feedback interface device.
Using an interface device, a user can interact with an environment displayed by a computer system to perform functions and tasks on the computer, such as playing a game, experiencing a simulation or virtual reality environment, using a computer aided design system, operating a graphical user interface (GUI), or otherwise influencing events or images depicted on the screen. Common human-computer interface devices used for such interaction include a joystick, mouse, trackball, steering wheel, stylus, tablet, pressure-sensitive ball, or the like, that is connected to the computer system controlling the displayed environment.
In some interface devices, haptic feedback or tactile feedback is also provided to the user, generally referred to as xe2x80x9cforce feedbackxe2x80x9d herein. These types of interface devices can provide physical sensations which are felt by the user manipulating the physical object of the interface device. For example, the Wingman Force joystick or Wingman Force mouse from Logitech may be connected to a computer and provides forces to a user of the controller. One or more motors or other actuators are used in the device and are connected to the controlling computer system. The computer system controls forces on the manipulandum of the force feedback device and/or on the housing of the device in conjunction and coordinated with displayed events and interactions on the host by sending control signals or commands to the force feedback device and the actuators.
In force feedback devices, it is important to have accurate control over the force output of the actuators on the device so that desired force sensations are accurately conveyed to the user. Typically, actuators are controlled by controlling the current through the actuator, such as a brushed DC motor or a voice coil actuator. To a first approximation, the torque output of the actuator is directly proportional to the actuator current. However, there are several different characteristics that make controlling current through the actuator difficult. These characteristics include the temperature variation of the coil in the actuator, back EMF (electromotive force) from user motion of the manipulandum of the device, power supply voltage variation, and variable coil impedance (which varies with temperature and current). The nonlinear force output response of actuators in relation to command signal level or duty cycle can cause problems in providing desired force magnitudes and sensations in force feedback applications, since the force magnitude that is commanded to the actuator may not necessarily be the force magnitude that is actually output by the actuator to the user.
In addition to these problems, there are some issues related to using a switch mode motor amplifier. The most significant of these issues for a force feedback system is related to the zero crossover point, i.e. the point where the current and actuator output force changes direction. The basic problem is that as the command to the actuator is modified from positive to negative (or negative to positive), there is a discontinuity as the force command passes through the zero point or near the zero point. This discontinuity in actuator current results in discontinuity in force output and is often quite perceptible to the user, leading to disruptions in the fidelity of the output force sensations in force feedback applications.
The present invention provides a method and apparatus for controlling an actuator to provide linear and continuous force output to a user of a force feedback device. The control features described herein allow discontinuities at zero-force crossover regions to be minimized while providing strong forces at the high ends of the force range and reduce the nonlinear force output produced by many actuators in relation to the input command signal to the actuator.
More particularly, a method for commanding a desired force from an actuator provided in a force feedback device includes providing two drive signals, each of the drive signals causing current to flow in a different direction in the actuator, thereby causing force to be output by the actuator in two different corresponding directions. When a desired force to be output is below a predetermined threshold force in a crossover region, the two drive signals are alternated for each period of the drive signals to cause a corresponding current in the actuator, thereby switching the direction of the desired force for each of the periods. When the desired force is above the predetermined threshold force, only one of said drive signals is used to cause current in the actuator in one direction and cause the desired force to be output in one direction.
The drive signals are preferably PWM drive signals and a select signal is preferably provided to select between using the two PWM drive signals and using only one of the PWM drive signals. Each of the PWM drive signals can control two switches in an actuator bridge circuit that provides current to the actuator. The select signal can be provided by a selection circuit including a flip flop and a parallel resistor, where the flip flop selects the PWM signal configurations. A circuit of the present invention for commanding a desired force from an actuator provided in a force feedback device includes a selection circuit and a bridge circuit performing similar functionality.
A method of the present invention for controlling an actuator to compensate for a nonlinear output of said actuator includes determining a desired command current for an actuator, where the command current causes a desired output force to be output by the actuator. The desired command current is correlated with an approximated point of a characterization curve of the actuator, where the characterization curve includes multiple points determined in a previously performed characterization of the actuator. The approximated point is determined between two successive points of the multiple points using a linear approximation, and a required command duty cycle is determined from the approximated point of the characterization curve. A drive signal having the determined command duty cycle is applied to the actuator to cause the actuator to output the desired force.
The multiple points of the characterization curve are stored in a look-up table, and preferably at least three points are stored to provide at least two linear sections of the characterization curve. The crossover region can also be implemented with the linearization method, so that alternating first and second drive signals are output to drive the actuator when the desired output force is between zero and a threshold force, where a linearly-approximated duty cycle is obtained for both of the drive signals.
Other features of the present invention can be implemented to provide a more linear force output. For example, voltage variations supplied to the actuator from a power supply can be compensated for such that the actuator output is linear regardless of the variations. A change in actuator current can be predicted that is caused by a back EMF effect induced by motion of a manipulandum of the force feedback device by the user. The predicted change in motor current can be based on a determined manipulandum velocity, and the drive signal to the actuator can then be compensated in accordance with the predicted change in current. Finally, the drive signal can be adjusted based on temperature information indicating a change in temperature of the wire coils of the actuator to compensate for a change in current in the actuator caused by temperature variation in the coil winding and to cause the desired force to be output.
The present invention provides methods and apparatus that control the actuator output in a force feedback device to allow a more linear and continuous output. The discontinuous current and output force occurring over a zero crossing of output force is minimized with the dual region control of the present invention. The nonlinear output of actuators is minimized using open loop precharacterization and linear approximation of actuator output and other techniques. These features allow the actuator in a force feedback device to provide force sensations with greater fidelity and realism.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following specification of the invention and a study of the several figures of the drawing.